Trained Eye


Insulating Knee Walls


We are trying to improve the house that our child has bought. It is a 55 years old, 11/2 story house with knee-walls and 4 small attics. Ice dams occurred at our first snow and melt this year.

One small attic runs along the entire length of the house and upon inspecting this area I noticed that there was an assortment of insulation stuck between the rafters. A definite no-no. The knee-walls themselves are studs with sheetrock and a total absence of insulation and vapour barrier. The area has a floor installed, however the space between the joists, below this floor, is totally devoid of vapour barrier and insulation. The attic, which is the roof ridge, appears to have "Fluff balls" installed but I have not yet had the opportunity to see for myself. The other side of the house has no access to the small attic areas and there have been small closets made into the attic of one bedroom. I have not seen these areas yet either.

We are going to upgrade the insulation and have some questions as to some of the things that will have to be done. First off, all the insulation presently between the rafters is going to be completely removed. Will we have to discard all this insulation or can some be salvaged? Is it worth salvaging the insulation? A vapour barrier will have to be installed. Due to the fact that we have a finished product, we can only work with what we have got.

Is it possible to run this vapour barrier over top of the 2x4 joists in an up and down fashion while going across the sheetrock? Do we need to cut pieces and somehow attach them to the 2 x 4's or joists with acoustic sealant?

 In one establishment we were advised that a vapour barrier is not necessary along the knee-wall. I disagree. Can we or should we apply extruded rigid pink foam to the outside of the knee-wall? Does this rigid material have to be covered? There is no overhang, thus no soffits. Will that make a difference as to how we insulate?

 On the CMHC site I found a passage that states that once the attic is sealed, it is not necessary to ventilate more and that attic ventilation is overrated. Perhaps I read this wrong. What kind of ventilation should there be for these small attics?

  The ridge attic has two gable-end vents. The end of the lower small attic is the sidewall of the house. Must this piece of outside wall, shaped like a triangle, also be insulated? Should the ridge attic indeed contain fluff balls for insulation? Should we remove this and set down new insulation? How should the access doors be insulated? In a house such as this there is an area that slopes from the knee-wall to the higher ceiling. What if anything can we do for that?

 Many questions to which we have no sure answers. We hope that you can possibly shed some light or give us a hint in the right direction with our dilemmas.

Thank you for taking the time to look at our questions and possibly provide us with some insight.


You have asked many specific and very relevant questions about proper insulating your Child’s home. Improperly insulated and sealed homes of this style will undoubtedly have problems with ice damming and often water leakage. I will try to answer many of your questions with some specific and some general answers that should help in your endeavour to improve the home in question.

The first item to address is your interpretation of the information on the CMHC website about air sealing. You have read this correctly, but this does not apply very well to older homes with the 11/2 story design that you are trying to improve. There are too many openings and junctions in the knee-wall areas to properly air seal this space with conventional methods, without major demolition. Any attempts to add polyethylene air-vapour barriers will be flawed, but may help somewhat. That is why increased insulation and ventilation, primarily in the knee-wall areas, will help considerably with the ice damming.

You are absolutely correct that the first course of action is to remove the old insulation from between the rafters in the knee-wall areas. If this is also present in the triangular gable ends of the knee-wall attics, as you have described, it should be removed, also. If the insulation is fibreglass or old mineral fibre batts and not damaged or mouldy, it could be reused between the main floor ceiling joists, which make up the floor of the knee-wall attics. Removal of the floorboards in this area may be necessary for installation but cutting out sections or drilling holes, to allow loose-fill insulation such as Cellulose Fibre to be blown in, may be easier. I would not be overly concerned with installing a proper Poly vapour barrier in this area, as it will not be continuous throughout the main floor ceiling below. Cellulose is a better material to use in this area, rather than Fibreglass, because it reduces air movement better. If the old plaster ceiling below is in reasonable condition and has several coats of paint, it will provide a partial vapour barrier, as well.

The last item I will address is proper insulation of the knee-walls. You should remove the existing “fluff balls” and install a poly air-vapour barrier over the wall covering and studs from the inside of the knee-wall attics. This should be caulked with acoustical sealant at the edges and top and bottom. Then, fibreglass batts or rigid foam insulation installed between the studs. The small attic access hatches can be easily insulated with rigid foam and weatherstrips, to provide a good seal. Once this is complete, a combination of roof vents and gable vents, installed on the triangular ends of the knee-wall attics, should improve the situation, dramatically. If the upper attic is well insulated and vented and the knee-wall attics, as well, the sloped sections in between are less of a problem. In an ideal situation, they should have their wall coverings removed and rigid foam insulation installed over the rafters before re-sheathing, but this may be difficult to accomplish.